1. Field of the Invention
The present invention relates to non-destructive optical measurement techniques, apparatus and systems for determining the active carrier profile in semiconductor layers. In particular it relates to using of optical energy to create charge carriers in these semiconductor layers and to probe changes in reflectivity created by these charge carriers as function of the depth in the semiconductor layer where these carriers agitate.
More particularly, the present invention relates to methods, apparatus and systems for extracting the active carrier profile in ultra shallow junctions in a particular semiconductor substrate. In particular it relates to extracting such information from a single set of measurements on a semiconductor substrate. The present invention also relates to devices and software for carrying out such methods.
2. Description of the Related Technology
In semiconductor processing, methods are required for the determination of properties of semiconductor materials, such as Si, SiGe, GaAs, . . . , and their dependence on processing conditions. Introducing species into a semiconductor material by, for example, ion implantation can change the properties of the bulk material. Other methods that can change the properties of the bulk material are manufacturing of the substrate, annealing such as for example rapid thermal processing (RTP) or rapid thermal annealing (RTA), etc. In CMOS (Complementary Metal Oxide Silicon) devices for example, it is important to be able to determine the junction depth and profile of the source and drain regions formed in the semiconductor substrate. For advanced high-performance CMOS technologies, it is, for example, crucial to be able to quickly and reliably characterize ultra shallow junctions. Especially, as CMOS structures, such as for example transistors, become increasingly smaller the doping profiles, in particular the active carrier profiles, shrink accordingly. Advanced CMOS structures will have gate lengths less than 50 nm and junction depths less than 70 nm. The exact determination of these profiles becomes more difficult and at the same time more critical. Process conditions need to be optimized in order to obtain the desired junction depth and profile and, hence, to yield the required device characteristics. One of the many crucial issues in fabricating state-of-the-art CMOS transistors is the precise control over the positioning and electrical characteristics of source/drain and extension regions. Besides the currently used low energy ion implantation and fast annealing techniques, much effort is placed in new techniques such as laser annealing (LTA) and low temperature Solid Phase Epitaxial Regrowth (SPER) to achieve higher concentration levels (above solubility) and steeper profiles (smaller thermal budget). Typically small variations in for example temperature or temperature gradient already cause unacceptable changes in for example junction depth.
Various methods exist to investigate the properties of the semiconductor active carrier profile. Some of these techniques, however, are destructive. Presently people use for doping characterization typically a combination of one-dimensional techniques such as Secondary-Ion-Mass-Spectroscopy (SIMS) for the total profile, Spreading-Resistance-Profile (SRP) for the electrically active carrier profile and Four-Point-Probe (FPP) measurement for sheet resistance. SIMS and SRP have the disadvantage that they are off-line techniques, applicable only on small pieces of material. In case of SRP the semiconductor substrate to be characterized is cleaved along a diagonal cleavage line and a two-point electrical measurement is then performed at subsequent positions along this cleavage line. For SIMS the material from the substrate under examination will be locally removed and subjected to further analysis. Furthermore a measurement on one specific position on a wafer takes about a day taking into account the sawing, preparation sample, measurement, calculation, etc. Conventional FPP can quickly measure whole wafers, but does not give any profile information and still requires rather large analysis areas, typically larger than 1 mm2. Furthermore, probe penetration leads to unreliable results on ultra-shallow profiles, particular when less than <30 nm deep. Recently some new promising techniques have emerged. For example two-dimensional carrier imaging techniques such Scanning-Capacitance-Measurement (SCM) or Scanning-Spreading-Resistance-Microscopy (SSRM), but one still needs small pieces for the measurements, a complicated and critical sample preparation is required and the depth resolution still needs improvement (5-10 nm). Furthermore these two-dimensional techniques depend critically on the availability of more reliable one-dimensional calibration profiles.
Other known techniques are non-destructive such as, for example, the Carrier Illumination (CI) technique, as disclosed in U.S. Pat. No. 6,049,220 and U.S. Pat. No. 6,323,951, and the Therma Probe (TP) technique, also called Thermawave technique or thermal wave technique as disclosed in “Non-destructive analysis of ultra shallow junctions using thermal wave technology” by Lena Nicolaides et al. in Review of Scientific Instruments, volume 74, number 1, Jan. 2003. All publications are hereby incorporated by reference in their entirety.
Referring to FIG. 1, in CI, TP and similar non-destructive optical techniques, typically two lasers (6, 3) are used. A first laser (6) is a focused pump laser or generation laser, generating a “pump” laser beam or generation beam. The first laser operates at a fixed wavelength, with an energy larger than the band gap of the semiconductor material under study. This pump laser (6) is used to generate an excess carrier profile in the bulk of the semiconductor material under investigation, giving rise to a depth dependent index of refraction of the material. Depending on the modulation frequency of the pump laser a quasi-static excess carrier profile is generated wherein the variation in the number of excess carriers is in phase with the variation of the pump laser or a dynamic excess carrier profile is generated wherein the variation in the number of excess carriers is not in phase with the variation of the pump laser. For the CI the frequency of the pump laser is in the kilo Hertz range, typically 1 kHz, resulting in quasi-static excess carrier profile, while for the TP the frequency of the pump laser is typically in the mega Hertz range, typically at about 1 MHz, resulting in a dynamic excess carrier profile dependent on the total carrier level as the lifetime of the excess carriers is inverse proportional to the total carrier level. The thus generated excess carriers distribute themselves in the semiconductor material according to a profile which is defined as the excess carrier concentration and is expressed in number of carriers per cm3 exceeding the level of carriers present within the semiconductor substrate without stimulation, this latter being labeled as the background carrier concentration or profile, e.g. in the absence of illumination. This background carrier concentration is dependent on the concentration of dopant atoms. Specifically, the excess carrier concentration changes from zero outside a surface of the semiconductor material to a finite value inside the semiconductor material. This change in excess carrier concentration results in a steep increase in the concentration of excess carriers at the surface of the semiconductor substrate. This steep increase of the excess carriers concentration at the interface between the semiconductor material under study and its surroundings, e.g. air, will be labelled as the near-surface component which will result in a near-surface component. As the depth z, which is defined from the illuminated surface of the semiconductor substrate into the semiconductor substrate, increases, the excess carrier concentration changes proportionally to the change in the concentration of dopant atoms or to the presence of recombination centers. For example, in some cases, the dopant concentration rises, but in other cases the dopant concentration dips first and then rises, depending on the detailed shape of the doping profile.
A reflected signal is generated by illuminating the optically stimulated semiconductor material with a second “probe” laser (3), generating a probe laser beam or probe beam, which may also be labeled analyzer beam, having a fixed wavelength which is typically higher (in case of CI) or lower (in case of TP) than the fixed wavelength of the “pump” laser. This probe laser beam will be reflected at the sample surface and/or at any region with a large change in the index of refraction proportional to the excess carrier profile, as is illustrated in FIG. 1. Reflected light (4) from the second laser (3) provides a signal, which is dependent on the profile depth. Currently reflected signals are converted to a value representative of junction depth using an algorithm developed through extensive correlation of CI or TP measurements with SRP measurements on a wide range of implants. FIG. 1 shows a semiconductor substrate (1), a pump laser beam (6) and a probe laser beam (3) impinging from the surroundings (2) on the semiconductor substrate (1). The incident probe laser beam (3) and the reflected probe laser signal (4) are indicated by respectively arrows (3) and (4). The semiconductor substrate (1) in this illustrative example comprises a doped layer (1a) formed on an undoped or lower doped region (1b). The substrate (1) can be formed by depositing an in-situ doped layer (1a) on top of layer (1b), yielding a uniform doping profile over region (1a) or can be formed by implanting dopants into the substrate (1), yielding a doped region (1a) and an undoped region (1b). By using e.g. ion implantation for implanting dopants into the substrate (1), any kind of doping profile can be obtained depending on the choice of implant species, the energy and implantation dose used. Layer 1a can be doped with a dopant of the same or the opposite type of dopant used to dope the underlying layer 1b. In FIG. 1, the excess carrier profile N(z) as function of depth z into the substrate (1) is also shown, indicated by graph 5. The probe laser beam (arrow 3) will be reflected, thus generating the reflected probe laser signal (arrow 4) at various positions on the semiconductor substrate (1). For example, the probe laser beam (3) may be reflected at the surface, yielding a surface component in the reflected probe laser signal (4). It may also be reflected by a change in the excess carrier profile which can occur at the surface, yielding a near-surface component, or at the interface between the doped part (1a) and undoped part (1b) on the gradient of N(z), yielding a bulk (or interface) component. Laser beams from both lasers, pump laser (6) and probe laser (3), are superimposed onto each other and may contact the semiconductor substrate (1) in the same or in a different area. Typically, both lasers are in a fixed measurement set-up and both incident laser beams have a direction perpendicular to the wafer surface or substrate surface, meaning incident at a zero angle relative to the wafer surface normal.
As indicated above, TP and CI use two lasers, a pump (830 nm for CI and 790 nm for TP) and a probe laser (980 nm for CI and 670 nm for TP). For activated structures the role of the pump laser is to generate a sufficient amount of excess carriers (typically more than 1018/cm3) varying with depth, such that the corresponding variations of the refractive index become visible for the probe laser and hence a sufficient contrast is obtained. The final excess carrier profile is a convolution of the generation, absorption and recombination mechanisms in the semiconductor substrate (1), where among others Auger recombination is strongly dependent on the underlying dopant profile. The latter contribution to the measured signal is called the electronic component. In addition there is a thermal component to account for, due to the local heating (5-15° K.) underneath the lasers caused by the high local energy densities (800 kW/cm2). The electronic and thermal components have opposite signs. For activated source/drain implants the amount of excess carriers in the highly doped region typically is rather low, typically by one order of magnitude relative to the substrate, to start rising steeply in the “junction” region towards the substrate level. Consequently, a significant part of the total reflected probe signal comes from close to the junction. This part of the signal is referred to as the interface component Einterface. Important to note is that this “junction” is not directly related with the metallurgical or electrical junction, but with a depth on a SIMS profile corresponding with a dopant level of about 1018 at/cm3, i.e. the excess carrier level in the substrate. The cosine shape of the reflected signal versus junction depth is due to the depth dependent constructive or destructive interference of the interface component with the reflection of the probe laser with the sample surface, called the surface component Esurface.
Due to the small size of the signals, typically 0.001% of reflection on pure silicon, a modulated pump laser needs to be used in combination with “lock-in” techniques. CI uses a low modulation frequency in the kilohertz range, typically 1 kHz, which corresponds to a quasi-static operation mode and the excess carrier profile is able to follow the modulation frequency of the pump laser. TP uses a high modulation frequency in the megahertz range, typically 1 MHz, causing wave formation and the excess carrier and temperature profiles will be out of phase with the pump signal as illustrated in FIG. 2a. A phase difference Φ exists between the pump signal (6) and the probe signal reflected (4) by the excess carriers and the induced temperature difference. In fact in practice one records respectively the in-phase (I=A.cos(φ)) and quadrature (Q=A.sin(φ)) components as shown in FIG. 2b, where A is the amplitude of the reflected signal (4).
For not-activated structures, a correlation between the reflected signals with the implanted dose has been established. As such, commercial TP/CI tools are being used in many important microelectronics companies and labs all over the world for the in-line qualitative monitoring of the reproducibility of implant and anneal cycles. Currently there is a tendency to use these qualitative analytical techniques in a more quantitative way. For an unknown sample, the depth where the interface signal originates from can, for a fixed pump laser power and corresponding excess carrier level, in principle be determined based from earlier established correlation curves plotting the amplitude Er of the CI signal or the Q component of the TP signal versus SIMS at the actual injection depth resulting in cosine-like shaped curves. Such correlation curves for CVD (chemical vapor deposition) grown layers indicate an achievable depth resolution of 1-2 Angstrom. A major problem with these correlation curves, however, is that they are dependent on many factors such as used implant species, type of implant/anneal process, etc. introducing a large uncertainty about which correlation curve(s) to use for an unknown sample.